In a carton monitoring system, a carton moves on a conveyor past a glue station where glue is applied to a portion thereof. After the glue is applied to the carton, the carton is folded and pressure is applied to the carton while the glue dries. Some time later, the cartons are filled with a product. If the cartons are improperly glued, the cartons may come apart, for example during shipping, and the product may be damaged or lost. Therefore, it is desirable to detect improper application of glue, e.g., glue skips and excess glue, and to mark or reject the improperly glued carton using a rejection device immediately after glue is applied to avoid individual inspection of the cartons.
Systems using a UV source and a UV sensor to detect a fluorescent material added to the glue have been proposed (see Edwards, et al. U.S. Pat. No. 4,704,603). However, prior art systems have several problems: the prior art systems are unable to synchronize the timing of the operation of the rejection device with the carton moving at line speed; the prior art systems cannot accommodate cartons of variable length; and the prior art systems do not operate when a distance between cartons varies.
Prior art systems also lack a dependable means for detecting and rejecting a double-stacked carton, a carton with a bent or removed flap, or a carton with an insert missing when the carton is moving at line speeds.